Method for determining the ignitability of a fuel

ABSTRACT

The invention relates to a method for determining the ignitability of a fuel, in particular a self-igniting fuel, based on the ignition delay of the fuel during the combustion in a test engine. According to the invention, the ignition delay is determined from the combustion curve and/or the heat curve.

The invention refers to a method for determining the ignitability of a fuel, especially of a self-igniting fuel, based on the fuel's ignition delay during combustion inside the test engine.

Standard DIN EN 590 stipulates the characteristic values and properties as minimum requirements for diesel fuels. Table 1 shows an overview of the important characteristic values for fuel.

Product Diesel (sulphur-free) [logo] Name Diesel fuel Holborn Europa Standard EN 590 (February 2000) Raffinerie GmbH Last updated on February 2002 Specifications Holborn item no. Diesel DIN EN 590 Parameter Unit min max min max Testing procedure Appearance clear, free of sediments and Visual soluble acid fractions (TAN) Color 2 ISO 2049 Density at 15° C. kg/m³ 820 845 820 845 EN ISO 12185 Flash point n.P.M. ° C. 59 >55 EN 22719 Sulphur content mg/kg 10 10 DIN EN ISO 14596 and DIN 51400 T11 Viscosity at 40° C. mm²/s 2.0 4.5 2.0 4.5 EN ISO 3104 Boiling pattern until 250° C. Vol % <65 <65 EN ISO 3405 Boiling pattern until 350° C. Vol % 85 85 EN ISO 3405 95 vol. % point ° C. 360 360 EN ISO 3405 Cetane number CFR test engine 51.0 51.0 EN ISO 5165 Cetane number BASF test engine 52.2 52.2 DIN 51773 Cetane index 46 46 EN ISO 4264 Total dirt accumulation mg/kg 24 24 EN 12662 Neutralization number mgKOH/g 0.2 0.2 DIN 51 558 Con. carbon. V. 10% dest. residue Weight % 0.3 0.3 EN ISO 10370 Copper corrosion (3 h 50° C.) Corr. degree 1 1 EN ISO 2160 Electrical conductivity pS/m 50 DIN 51 412-2 Oxidation stability g/m³ 25 25 EN ISO 12205 Cloud point Summer product ° C. 5 EN 23015 Transition product ° C. −3 Winter product¹ ° C. see driveability limit CFPP² Summer product ° C. −2 0 EN 116 Transition product ° C. −13 −10 Winter product¹ ° C. see driveability −20 limit Ash content Weight % 0.01 0.01 EN ISO 6245 Water content mg/kg 200 200 EN ISO 12937 Oiliness (HFRR WS 1.4) μm 460 460 ISO 12156-1 Polyaromatic compounds Weight % 11.0 11.0 IP 391/95 Filtering capacity min 2 SEDAB Test Delivery times Summer product 04/15-09/14 04/15-09/30 Transition product (autumn) 09/15-10/31 10/01-11/15 Winter product¹ 11/01-02/28 11/16-02/28 Transition product (spring) 03/01-04/14 03/01-04/14 Driveability limit³ Cloud point ° C. −5 −6 −7 −8 −9 DIN EN 23015 CFPP ° C. −30 −28 −25 −23 −22 DIN EN 116 Remarks: Sampling done according to DIN 51750 T1 + T2 ¹premium diesel (11/01-02/28) with at least 150 mg/L WASA/L diesel ²to a maximum of 500 mg MDFI/kg diesel ³defined through the combination of cloud point after short sedimentation test and CFPP

Especially important in all of this is ignitability, described with the cetane number or CN. Briefly explained, one can say that the ignitability of a diesel fuel has an important impact on the engine's combustion process and with it on noise and emissions. In principle, the higher the cetane number, the shorter will be the time elapsed between fuel injection and start of combustion (ignition delay). Consequently, the pressure increase speed decreases and with it the combustion noise. Maximum pressures and temperatures also become lower, something that has a positive effect on nitrogen oxide emissions. In a cold start, a higher cetane number has a favorable impact on HC emissions.

The cetane index given in the standard is alternatively calculated from density and boiling range and is only partially correlated with the CN numbers obtained from the engine because the behavior of ignition accelerators is not taken into account. The CN numbers are determined empirically in special test engines. So the fuel to be tested can be adapted, the compression ratio in the CFR engine and the air intake in the BASF engine can be varied.

The objective is to compare the ignitability of the fuel to be tested with fuels with known cetane numbers and, if need be, to determine the cetane number through interpolation. In the standard, cetane (n-hexadecane) was arbitrarily assigned the cetane number of 100 and alpha-methyl naphthalene the cetane number of 0. By mixing the components, one can produce a fuel that will have the same ignitability as the fuel to be tested. The cetan number sought will then correspond to the volumetric share of cetane in the fuel mixture.

To determine the ignitability of a fuel, the test engines are operated in accordance with Table 2.

TABLE 2 Dimensions and measuring conditions of the BASF test diesel and the CFR test diesel Description BASF test diesel CFR test diesel Construction Technical testing stand of the Cooperative Fuel Badische Anilin- & Soda Fabrik Research Committee of AG the American Society of Automotive Engineers Manufacturer Hermann Ruf, Mannheim Waukesha Motor Co., Waukesha, Wisconsin, USA Boring mm 90 S 2.6 Stroke mm 120 114.3 Displacement cm³ 850 613 Compression Fixed 18.5:1 Adjustable Volumetric control   6-21:1 Mode of operation Swirl chamber Swirl chamber Measuring procedure and measuring conditions Procedure Const. ignition delay Const. ignition delay RPM rpm 1000 ± 10  900 ± 9  Intake air temp. ° C. 20 ± 5 66 ± 1 Coolant temp. ° C. 100 ± 2  100 ± 2  Oil temperature ° C. 70 ± 5 57 ± 8 Injection start °CA b. UDP 20 13 Ignition start °CA b. UDP 0 0 Injected quantity cm³/min   8 ± 0.5   13 ± 0.2 Ignition delay display Electronic ignition delay meter Ignition delay meter Standard DIN 51 773 ASTM-D 613 62 T Measuring range CaZ 30-100 30-100 ¹Manufacturer: Hermann Ruf Co., 68 Mannheim-Neckarau ²Manufacturer: Waukesha Motor Co., Waukesha, Wisconsin, USA

In the CFR engine, the injection point has been fixed to 13° (crank angle before upper dead point (° CA b. UDP)). The compression ratio is varied in such a way that combustion always starts in the UDP, i.e. with a 13° CA ignition delay. A sensor impinged on the cylinder pressure determines the start of combustion. In this case, it is assumed that the start of combustion can be exactly determined with this sensor and the analog processing of the signal. Own tests have shown, however, that the curve of the cylinder pressure is only of limited use for determining the start of combustion. In this context, FIG. 1 shows the pressure curves of diesel engines with different combustion processes. According to the principle, the DI engine has the highest pressure increase speed and even the start of combustion is in the 4-2° CA crank angle before UDP range can still be determined relatively precisely from the pressure. In the swirl chamber engine, however, the allocation of the start of combustion becomes significantly harder because of the already slower energy turnover. This problem also applies especially to the test engine used in a standardized way, as it is executed as a swirl chamber engine too.

Compared to different test engines, the standard gives the accuracy of the process in the 2.8-4.8 CN range. The repeat accuracy lies between 0.8 and 1 CN. Operation is manual and lasts 20-30 minutes per cetane number.

Tests were (and still are being) performed to determine the cetane number with another instrument (especially with vegetable oils) outside of the engine. Thus, the Ignition Quality Tester (IQT) of the Advanced Engine Technology Co. of Ottawa, Ontario, and the Fuel Ignition Tester of the Fueltech AS Co, of Trondheim, Norway, are used mostly in Canada and the USA. Both measuring instruments determine the ignitability along the measured ignition delay of the fuel in a constant volume, heated high-pressure chamber. Automobile manufacturers are skeptical about the standardization of these processes that take place outside the engine. A fundamental improvement of the engine process has not been found.

The following problem areas have been detected in assessing the standard processes for determining the cetane number:

-   -   The accuracy of the process can be improved upon.     -   The process is time-consuming; cannot be automated and allows no         online display.     -   In the process, the start of combustion (ignition delay) is         determined from the cylinder pressure after analog processing         and displayed directly. No exact calculation of the start of         combustion takes place.     -   An evaluation of the cetane numbers of vegetable oils with the         needed accuracy has so far been impossible.     -   The injection moment is set mechanically. There is no         operational check with a needle stroke display.     -   The combustion process in the swirl chamber is no longer         contemporary.

It is therefore the task of the invention to suggest a process that will make a fast and reliable characterization of the ignitability of fuels possible.

The task is solved by determining the ignition delay based on the combustion curve and/or the heat curve (heat input through combustion). It is especially suggested to calculate the combustion curve and/or heat curve and there from the actual start of combustion, which must be known precisely for determining ignition delay, with the laws of thermodynamics (see FIG. 2), The combustion curve and/or heat curve can finally be calculated with mathematical methods based on the first law of thermodynamics (see FIG. 2) known to the expert. For this, an exact metrological record of cylinder pressure as a function of time or test engine crank angle and, generally, the known caloric data of the fuel are needed.

As input data—and apart from the cylinder pressure—the mass in the cylinder (injected fuel mass and mass of the combustion air supplied) must be metrologically recorded. The final output is in the end the combustion curve and/or heat curve corresponding to the heat input through combustion, although both magnitudes are different from one another merely by the different way in which the wall heat losses of the test engine were considered. The combustion curve and/or heat curve calculated in such a way—that can also be shown as change in the fuel mass within the cylinder (dm_(B)) as a function of the crank angle (see FIG. 3)—is significantly steeper than the curve of the pressure at the start of combustion and therefore more suitable for determining ignition delay.

To improve the accuracy of the process, it is necessary to determine the start of injection apart from the start of combustion. To determine the start of injection, it is recommended to equip the injector with a needle stroke sensor so the lifting of the injector needle is recognized and the actual start of injection can be measured exactly and independently from other parameters. If the injector is electromagnetically or piezoelectrically controlled, it is also possible, as an alternative, to determine the needle stroke from the control signal (voltage measurement and consideration of additional injector performance characteristics, if applicable) instead of using the needle stroke sensor.

A built-in piezoelectric pressure sensor (like the one typically employed in the R&D tasks of engine developers) is used for measuring cylinder pressure. The signal is amplified and converted to a voltage signal proportionate to the cylinder pressure (FIG. 4). The voltage signal is fed to a fast measurement data recording system, where it is digitalized, further processed and stored. In this case, the recording of the pressure data is done based on time and/or crank angle. A corresponding sensor is intended for placement on the crank shaft (FIG. 4). The air mass sucked in must be determined with an air mass meter; the injected quantity (=fuel mass) is fixed and can be determined in preliminary tests done on a pump testing stand. The combustion process should be advantageously adapted for direct injection.

Examples of the process steps are listed below:

-   -   Crank angle-based measurement of the cylinder pressure signal         (scanning frequency: 0.2° CA)     -   Measurement of the air mass sucked in.     -   Measurement of the needle stroke (scanning frequency: 0.2° CA)         for determining the start of injection.     -   Calculation of an averaged needle stroke and pressure curve from         300 working examples.     -   Calculation of the combustion curve and/or heat curve.     -   Calculation of the ignition delay from the combustion curve         and/or heat curve and the corresponding start of injection.     -   Correlation of results with the known standard fuels or their         mixtures or also with previously analyzed fuels of known         composition and/or ignitability.     -   Calculation, display and storage of the cetane number         determined.

The important advantages of the device and process include:

-   -   Higher accuracy, especially by calculating the combustion curve         and heat curve, respectively     -   Higher accuracy because the needle stroke is measured.     -   Higher repeat accuracy by obtaining the average from approx. 300         working examples     -   Possibility of automation.     -   Online display of the cetane number.     -   Automatic or half-automatic execution.     -   Measurement with regular gasoline no longer needed, as the         results can be made available in a database. The compression         ratio can be exactly calculated from the pressure curve before         the moment of ignition.     -   Current DI combustion process.     -   Can be used with vegetable oils.

Modifications of the invention are easily possible within the framework of the patent claims, in which case it is expressly mentioned that all individual characteristics published in the patent claims, in the description and in the figures can become reality in any combination thereof as far as this is possible and makes sense. Thus, for example, a previously defined value for the combustion curve and/or heat curve such as the start of combustion can be determined with the help of a sensor, especially one for measuring an ionic current inside the test engine, a sensor for measuring the structure-borne noise of the test engine and/or an optical sensor. It can likewise be advantageous for the determination of ignitability to include a statistical analysis of one or several of the measured values mentioned so far. In this case, it is once again advantageous if the statistical analysis includes the recording of measured values of several successive working cycles. The statistical analysis can also encompass the recording of one or several measured values (for example, of 200 to 300 working cycles) of the test engine, especially in a defined operational point of the test engine. In this way, many different readings are obtained from which averages can be calculated so that the ignition delay can be determined with a great deal of accuracy. 

1. Process for determining the ignitability of a fuel, especially of a self-igniting fuel, based on the ignition delay of the fuel during combustion inside a test engine characterized in that the ignition delay is determined based on the combustion curve and/or the heat curve.
 2. Process according to claim 1 characterized in that at least one previously defined value of the combustion curve and/or heat curve such as the start of combustion, for example, is determined with the help of at least one sensor, especially a sensor for measuring the ionic current inside the test engine, a sensor for measuring the structure-borne noise of the test engine and/or of an optical sensor.
 3. Process according to claim 1 or 2 characterized in that the ignition delay of the fuel is compared with the ignition delay of at least one more fuel with known ignitability, especially that of a standard fuel, so that the ignitability is determined from this.
 4. Process according to the previous claim characterized in that cetane (CN 100), alpha-methyl naphthalene (CN 0) and/or heptamethylnonane (CN 15) are used as standard fuel.
 5. Process according to one or several of the previous claims characterized in that a characteristic number that describes the ignitability is determined based on the state variables of pressure and/or temperature of the gases being produced during the combustion and/or the dwelling time of the fuel.
 6. Process according to one or several of the previous claims characterized in that the ignitability is converted to a corresponding cetane number.
 7. Process according to one or several of the previous claims characterized in that the start of combustion, the duration of combustion, the central combustion zone and/or the maximum combustion speed are determined from the combustion curve and/or the heat curve and, based on at least one of these parameters and the moment of injection of the fuel into the test engine, the ignition delay is determined.
 8. Process according to one or several of the previous claims characterized in that the cylinder pressure is detected during the combustion of the fuel in the test engine and used for determining the combustion curve and/or the heat curve.
 9. Process according to the previous claim characterized in that the cylinder pressure is detected as a function of the time and/or the crank angle of the test engine cylinder.
 10. Process according to one or several of the previous claims characterized in that the cylinder pressure is detected with the help of a piezoelectric cylinder pressure sensor.
 11. Process according to one or several of the previous claims characterized in that the crank angle of the test engine is determined with the help of a crank angle sensor, especially as a function of time.
 12. Process according to one or several of the previous claims characterized in that the start of injection of the fuel into the test engine cylinder is used for determining the ignition delay.
 13. Process according to the previous claim characterized in that the start of injection is metrologically recorded.
 14. Process according to the previous claim characterized in that the start of injection is recorded with the help of a needle stroke sensor.
 15. Process according to one or several of the previous claims characterized in that the ignition delay is calculated from the difference of the start of injection and the start of combustion and/or further turnover points characteristic for combustion.
 16. Process according to one or several of the previous claims characterized in that the combustion curve and/or heat curve is/are calculated as a function of the test engine crank angle.
 17. Process according to one or several of the previous claims characterized in that the combustion curve and/or heat curve is/are calculated from the measured data from one test engine working cycle.
 18. Process according to one or several of the previous claims characterized in that the combustion curve and/or heat curve are calculated based on the measured data of several test engine working cycles, whereby the corresponding averages of the respective measured data, in particular, are calculated from the measured data from several working cycles.
 19. Process according to one or several of the previous claims characterized in that the fuel is injected into the cylinder by direct injection or using the side chamber process.
 20. Process according to one or several of the previous claims characterized in that the volume flow and/or the mass of the combustion air flowing into the test engine and/or its change as a function of time and/or of the crank angle is/are used for determining the combustion curve and/or the heat curve.
 21. Process according to one or several of the previous claims characterized in that the pressure increase speed is taken into account during combustion of the fuel in the test engine when determining the ignitability.
 22. Process according to one or several of the previous claims characterized in that the determination of the ignitability includes a statistical analysis of the measured data.
 23. Process according to the previous claim characterized in that the statistical analysis encompasses the recording of measured data of several successive working cycles.
 24. Process according to claim 22 characterized in that the statistical analysis encompasses the recording of measured data from one or several working cycles, for example between 200 and 300 test engine working cycles, especially in a defined operational point of the test engine.
 25. Process according to one the claims 22 to 24 characterized in that the statistical analysis includes the calculation of averages of the measured values.
 26. Process according to one or several of the previous claims characterized in that the test engine compression ratio is calculated based on the cylinder pressure, especially in a defined crank angle.
 27. Process according to one or several of the previous claims characterized in that the calculations and the control of the individual process steps take place automatically with the help of a control unit.
 28. Process according to one or several of the previous claims characterized in that the measured values and/or the determined ignitability is/are displayed online. 